1. Field of the Invention
The invention relates to a method of producing from extrudable plastic an internally smooth tube having externally circumferentially extending ribs, the molten or melt plastic being extruded from a stationary nozzle into the interior of a moving mold passage having the desired outer form of the tube and being pressed against the inner wall of the mold passage, the smooth inner wall of said tube being formed by means of a mandrel provided in the mold passage. The extrudable material will usually be a thermoplast, such as in particular PVC or PE. It may, for example, also be crosslinkable PE.
2. Description of the Prior Art
Methods of the type outlined above are widely known. For example German patent specification No. 2,450,171 describes such a method in which the extrusion passage opens axially directed into the interior of a moving mold passage. At a considerable distance from said opening the nozzle core carries the piston, the extruder-side end wall of the piston being formed as slightly rising cone. The melt then flows against said cone, is expelled thereon outwardly and should press itself into the ribs-forming depressions in the inner wall of the moving hollow mold. Since the extrusion pressure is not directed outwardly (the melt flows freely between nozzle and cone) the force pressing the melt into the rib cavities is obtained only by the entraining effect of the moving mold on the viscous-elastic melt pressed between the moving mold and the mandrel. This not only leads to a relatively high pressure over a long distance., a further disadvantage resides in that the melt tends to form shear surfaces along cylinder lines in the interior of the tube. The main disadvantage of this method is, however, that it is extremely difficult to achieve a satisfactory formation of the ribs For the latter an extremely accurate coordination is necessary between the extrusion velocity, the withdrawal velocity and the extrusion temperature. Also, the melt must satisfy a great number of parameters.
According to another known method (German patent specification No. 2,709,395) a "melt tube" (a tube-shaped melt) emerges floatingly from the extrusion nozzle and is pulled onto a mandrel held spaced apart from the nozzle by an extension of the nozzle core. There, the flexible melt tube, which is widened over the likewise conical extruder-side end wall of the core, comes into the "gripping region" of the moving mold, is gripped by the latter, so that the deformable melt is pressed to a greater or lesser extent into the outer grooves of the mold passage under the influence of the entraining force. In this case the problems which are encountered are substantially the same as those in the method first described.
A further method of the type outlined is known from European patent specification No. 0 142 482. In this method a flexible tube or hose is also extruded in the axial direction and is inflated by means of compressed air in the interior of the mold passage in order to be pressed into the depressions of the mold forming the ribs. Considerable problems are encountered here in achieving satisfactory filling of the rib cavities simply by the compressed air. The nozzle core carries a second extrusion head from which a further plastic tube or hose is extruded which is also inflated by compressed air and pressed from the inside against the more or less smooth wall of the already produced ribbed tube. The core of said second extrusion head in turn carries in spaced relationship a piston which is intended to smooth the inner wall of the tube thus produced. In this method the problem of satisfactory formation of the ribs is present to a greater extent.
Finally, German Auslegeschrift No. 2,362,444 discloses a method in which in the interior of the moving mold a slightly conically outwardly directed tubular plastic melt stream is extruded into the annular space between the nozzle end wall, the mandrel and the mold passage. In this case extrusion is thus directed into the space to be filled. In this method as well, particularly when it is to be carried out with reasonably acceptable operating speeds, the formation of a satisfactory rib is difficult. Furthermore, with this method a pulsating backpressure occurs due to the different material requirement in the ribless region on the one hand and the ribbed region on the other and this back-pressure is also disadvantageous. Finally, in the known method the inner surface of the tube is very often irregular.